The role of ultraviolet light (UVL) and sunlight in the etiology of certain types of skin cancer now seems well established. Some investigators have speculated that the carcinogenic activity of light depends upon the photochemical conversion of sterols, such as cholesterol, into carcinogenic substances. Since the demonstration of the occurrence of a naturally occurring carcinogenic sterol in UVL- radiated skin, this laboratory's efforts have centered around the elucidation of the role this compound plays in the development of actinic skin cancer. A correlation between the levels of compound formed in skin of hairless mice upon chronic suberythemic exposure to UVL and the onset of skin tumors has been found. Furthermore antioxidants have been shown to inhibit the formation of the carcinogen in vitro and preliminary evidence suggests they also suppress the induction of skin tumors. This proposal is designed to determine whether additional information will implicate the compound in the etiology of UVL-carcinogenesis and involves: 1) Studies on the role of antioxidants in suppression of UVL- carcinogenesis; 2) studies on the metabolism and detoxification of the carcinogen; 3) the ability of the carcinogenic compound to transform normal cells, in cell culture, to cells with malignant potential and; 4) the ability of the carcinogen to induce tumors when applied, in vivo, to mice. Should the carcinogen induce transformation in cell culture and initiate tumors in vivo, then an overwhelming body of evidence implicating the compound in the etiology of skin cancer will have been amassed. Studies of the metabolism of this compound may indicate means by which its formation can be prevented.